1. Field
This application relates generally to vortex generation over aircraft wing surfaces.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Many swept wing aircraft suffer from aerodynamic buffet or asymmetric wing stall at high angles of attack (AOA) or high “alpha”. Alpha is measured in units of degrees between a lifting body reference line, which is often the chord line of an airfoil, and a vector representing relative motion between the lifting body and the fluid through which the lifting body is moving. In the example given above, an angle-of-attack of 8 degrees would represent an angle of 8 deg between the chord line of the airfoil and the relative velocity vector of the airflow.
Under high alpha conditions, the upper surfaces of swept wings may experience reduced energy airflow, which may result in unsteady flow separation (stalling) and/or unsteady shock wave formation. This is known to be a cause of buffet and/or asymmetric stall problems that can result in poor aircraft handling qualities at high alpha.
Buffet and/or asymmetric wing stall may be induced by unsteady transonic tip shockwaves at combinations of transonic Mach numbers and high alpha. An unsteady transonic tip shock can occur when moderately high alpha, low-energy, separated flow conditions destabilize the normally-occurring wingtip shockwave on the upper surface of a wing. This phenomena may affect as much as a spanwise approximate outer half of aircraft wings. Because fighter aircraft tend to be operating at relatively high alpha when their pilots are attempting to track another aircraft for weapons employment, transonic tip shock-induced buffet can severely degrade weapons targeting. This is especially true for an aircraft that experiences buffet over an alpha range that includes the optimum sustained turn condition for the aircraft, e.g., an alpha value at which the aircraft can maintain a maximum turn rate without losing airspeed.
Such phenomena, being related to unsteady shock/boundary layer interaction issues, can be difficult to predict using known analytical tools such as computer modeling and wind tunnel testing. As a result, the propensity of a new aircraft to experience buffet and asymmetric stall problems at high alpha is often not detected until development has reached the stage of flight testing. At this point in a new aircraft's development, extensive airframe configuration changes necessary to reduce buffet and stall issues can be extremely difficult and expensive to make. However, there are a few relatively simple and inexpensive approaches to mitigating the problem with only minimal modifications being made to the existing structure of the aircraft. One such approach is the addition of vortex generators. Careful leading edge and trailing edge flap adjustments (scheduling) for different airspeeds and angles of attack may also mitigate a buffet problem, but oftentimes will not provide a complete or satisfactory solution on their own. Various types of vortex generators, snags, and/or strakes may be attached to or built into a wing's leading edge to energize airflow over the wing at high angles of attack, but such structures come with a significant drag penalty.
There are several commonly known leading edge lift augmentation devices that are incorporated into aircraft wing structures to improve an aircraft's performance at low airspeeds and/or high angles of attack. Slats, for example, are airfoils carried by main wing bodies of aircraft wings and deployed ahead or upstream of leading edges of the main wing bodies. Slats are thus positioned to allow air to flow through a slot between the slats and the main wing body leading edges to energize airflow over the wings at high angles of attack. Other leading edge lift augmentation devices, such as leading edge flaps, may be deployable in such a way as to effectively extend the leading edge of a wing forward and/or down. Yet other leading edge lift augmentation devices may be deployable in such a way as to rotate a portion of a wing's lower surface down and forward. Each of these leading edge configurations increases lift by helping the airflow turn as it encounters the wing at elevated angles of attack, thereby reducing or delaying flow separation.
Unfortunately, while leading edge lift augmentation devices greatly improve a wing's performance at low speeds and/or high angles of attack, and while vortex generators can be used to mitigate buffet under such conditions, both the leading edge lift augmentation devices and vortex generators tend to increase drag. Slats and flaps can be retracted flush with a wing's mold line to reduce drag, but vortex generators are generally fixed in place, and are thus seldom seen in an optimum size or configuration on high-performance aircraft. The use of vortex generators on low observable aircraft is also limited by a characteristic lack of low-observable compatibility.